The present invention describes a method for variation of a rated current which is specific for a motor, by provision of a rated-current basic-value signal as a function of a rated-current basic value Irated of the motor. It is also intended to provide a corresponding apparatus for variation of a rated current.
During operation of a motor, energy is converted to heat via the electrical losses resulting from the current (copper losses) and the magnetization changes in the motor (iron losses), as well as the friction that occurs. If the amount of energy converted is greater than the amount of thermal energy which is passed to the environment, then there is a risk of the motor overheating. Attempts to prevent thermal destruction of a motor by means of a warning system having one or more sensors have shown that this method does not ensure adequate protection. In this case, even the fitting of one or more sensors to the winding of a motor has been found to be problematic.
According to the prior art, a method is therefore offered that is known by the name I2t monitoring. In this case, a current level which is referred to as the rated current or S1 current is defined for a motor, representing the highest value of the motor current level at which the motor can still be operated indefinitely without any need to be concerned about overheating of the motor.
If the rated current Irated of a motor is known, then the load level Xcon on the motor when the motor is loaded for a relatively long time with an undefined, constant motor current level Icon can be calculated using the following equation, Equation 1:
                              X          con                =                              I            con            2                                I            rated            2                                              (                  Equation          ⁢                                          ⁢          1                )            
If the current level changes from the value I2 to the value I2 at the time t=0, then the thermal time constant τth of the motor must also be taken into account in the calculation of the load level X(t) of the motor at the time t≠0. This is done using the following formula:
                              X          ⁡                      (            t            )                          =                                            I              2              2                                      I              rated              2                                -                                    (                                                                    I                    2                    2                                                        I                    rated                    2                                                  -                                                      I                    1                    2                                                        I                    rated                    2                                                              )                        ·                          ⅇ                              t                                  τ                  th                                                                                        (                  Equation          ⁢                                          ⁢          2                )            
For an approximation to the abovementioned equation 2, the series development of the exponential function is truncated after the second term. Equation 2 then becomes:
                              X          ⁡                      (            t            )                          ≈                                            I              1              2                                      I              rated              2                                -                                    (                                                                    I                    2                    2                                                        I                    rated                    2                                                  -                                                      I                    1                    2                                                        I                    rated                    2                                                              )                        ·                          t                              τ                th                                                                        (                  Equation          ⁢                                          ⁢          3                )            
If the load level Xcon or X(t) reaches a value of 100%, then remedial measures must be taken in order to avoid overheating of the motor. This is done, for example, by emission of a warning to a motor user, by automatically shutting down the motor, or by automatically reducing the motor current level to, for example, 90% of Irated.
However, the method described here for calculation of the load level on a motor according to the prior art offers only inadequate protection against overheating of a motor since this calculation of the load level on the motor does not take account of environmental influences.